The present invention relates to an image recording apparatus such as an electrophotographic copier having a toner concentration detecting circuit by which a change in magnetic permeability of a two-component developer made of toner and carrier is detected so that toner concentration in the developer can be known.
In an image recording apparatus such as an electrophotographic copier and a facsimile, a document is irradiated by a light beam, and an image is recorded on a recording sheet by reflected light from the document, or by an electric signal of the image sent from the outside.
In this type of image recording apparatus, a visual image is formed using the two-component developer made of toner and carrier, and is recorded on the recording sheet. In the two-component developer, a toner mixing ratio to a predetermined carrier weight is 4 to 10 weight %. The toner is decreased when recording is conducted, and therefore, it affects density of a copied image. Accordingly, in order to obtain a recorded image with a constant density, it is necessary to detect toner concentration in the developer, to supply toner to the developer when the toner concentration is decreased, and to control the toner concentration so that it is within an appropriate range.
Conventionally, the following methods are widely known to detect toner concentration: a method in which a toner image density is optically detected according to a small piece having reference density; and a method in which a toner concentration detecting sensor including a coil is provided close to the developer, and magnetic permeability of the developer is measured by the sensor so that the toner concentration (toner mixing ratio) in the developer can be known. The method, by which magnetic permeability is measured, has the advantages that the magnetic permeability can be always measured, and it is not necessary to change copying processes. The detected toner concentration is compared with a toner concentration reference value, and the toner is supplied so that the toner concentration in the developer can be always maintained.
FIG. 6 is a block diagram showing circuits by which the toner concentration is detected from the magnetic permeability of the developer, and which are applied to a developing section of a conventional image recording apparatus. In the drawing, numerals 51Y, 51M, 51C, 51K are toner concentration detecting sensors which are provided in developing devices in which yellow (Y), magenta (M), cyan (C), black toners are respectively loaded, and by which toner concentration is detected according to the change of an oscillation frequency depending on the magnetic permeability of the developer. Numeral 52a is an analog multiplexer by which one of signals sent from the plurality of sensors 51Y to 51K is selected by a selection signal sent from a CPU 50, and sent to the next circuit, and numeral 53b is a waveform correction circuit in which a Schmitt circuit is used. Numeral 54a is a counter, for example, a 9-bit counter, which counts pulses in a predetermined time by a gate signal sent from a gate signal generating circuit 55a. Numeral 55b is a resonance circuit using a ceramic oscillation element. Numeral 58 is a D/A converter. Numeral 59 is a sample/hold circuit which samples and holds an analog value inputted by the sample/hold signal sent from the gate signal generation circuit 55a. Numeral 60 is a differential amplifier, and numeral 61 is an A/D converter accommodated in the CPU 50.
Outputted values from sensors 51Y to 51K are increased when a frequency is increased as the toner concentration of the developer is increased. One of the outputs is selected by the analog multiplexer 52a, waveform-corrected through a waveform correction circuit 53b, and after that, the number of pulses in a predetermined time is counted by the counter 54a. The counted value (digital value) is converted into a voltage value by the D/A converter 58, and after that, the voltage value is held by the sample/hold circuit 59. The held voltage value is inputted into the differential amplifier 60 together with the voltage value into which a digital value outputted from the CPU 50 is converted by the D/A converter 58, wherein the digital value is outputted from the CPU 50 to correct deviation due to the kind of the developer or an intrinsic error of the developing device. After that, the voltage value is amplified by the differential amplifier 60 and inputted into CPU 50. The inputted value is converted into a digital value by A/D converter 61 in the CPU 50, and after that, it is compared with a reference value. A toner replenishing direction signal is outputted to a toner replenishing means 17 corresponding to the difference between the above two values, and the toner is replenished to the developer.
In the toner concentration detecting circuit of a conventional image recording apparatus as described above, sensitivity of a sensor, that is, a change of the oscillation frequency of the sensor to a change of toner concentration of the developer, is extremely low. The sensitivity is as follows: when a center frequency of the sensor is 200 KHz, the frequency increases by 0.8 KHz as the toner concentration increases by 1 wt %. That is, when a gate time is 10 ms, the frequency increases by 8 pulses with respect to a 1% increase of the toner concentration. Accordingly, a change of the toner concentration is 0.125% per pulse, so that a highly accurate circuit is necessary.
However, as described above, in the conventional toner concentration detecting circuit, the circuit is complicated; noises are caused and an error becomes large because A/D conversion is further carried out after D/A conversion; and the cost becomes high, which are problems.
Further, because many analog circuits are used in the detecting circuit, an error in the circuit becomes large when voltage of a power source or temperature is varied, or variations in parts are caused.